1. Field of the Invention
This invention relates to a process for recovering oil from a subterranean formation by placing fluid into the formation by means of a well. More specifically, this invention relates to a method of designing a microemulsion for recovering oil from an oil-bearing subterranean formation.
2. Description of the Prior Art
The petroleum industry has recognized for many years that only a small fraction of the original oil in place in a reservoir is expelled by natural mechanisms. It is also well-known that conventional methods of supplementing natural recovery are relatively inefficient. Typically, a reservoir may retain half its original oil even after the application of currently available methods of secondary recovery. Accordingly, there is a continuing need for improved recovery methods which will substantially increase the ultimate yield of petroleum for subterranean reservoirs.
Waterflooding is by far the most widely practiced method for recovering oil from a formation after naturally occurring forces in the formation have declined in their ability to expel oil. In waterflooding, water is injected through an input well to drive oil to offset producing wells. Much of the current work in oil recovery technology has been directed toward improving the efficiency of waterflooding processes or developing alternative processes.
Surface-active agents or surfactants are one class of materials which have been proposed for improving the efficiency of waterflooding processes. Much of the oil that is retained in the reservoir after a typical waterflood is in the form of discontinuous globules or discrete droplets which are trapped within the pore spaces of the reservoir. It has been suggested that, because the normal interfacial tension between the reservoir oil and water is so high, these discrete droplets are unable to sufficiently deform to pass through narrow constrictions in the pore channels. When surface-active agents are added to the flood water, they lower the interfacial tension between the water and the reservoir oil and permit the oil droplets to deform and flow with the flood water. It is generally conceded that the interfacial tension between the flood water and the reservoir oil must be reduced to less than 0.1 dyne/cm for effective recovery.
Where conventional surfactant waterflooding may be effective in obtaining additional oil from subterranean oil reservoirs, it has a number of shortcomings which detract seriously from its value. One difficulty which has been observed in the use of surfactants in general is the tendency of the surfactants to be depleted from the injected solution. It has been postulated that at least a part of the surface-active agents may be adsorbed on the rock surface of the reservoir or physically entrapped within the pore spaces of the rock matrix. It is also known that many surfactants react with ionic substances in the water within the formation and are precipitated and therefore unable to interact at the oil/water interface to reduce the interfacial tension. As a result the oil recovery efficiency can be reduced due to surfactant depletion.
One method for reducing surfactant depletion and increasing oil recovery is the use of microemulsions. Microemulsions are well-known (see for example, U.S. Pat. No. 3,254,714, Gogarty et al., issued June 1966) and are mixtures of a liquid hydrocarbon, water, and a surfactant. Optionally, a cosurfactant, such as an alcohol, and electrolytes may be present in the mixture. The microemulsion is a stable, transparent or translucent mixture of these compounds and is injected into the formation and displaced through the formation by means of a driving fluid such as thickened water. These microemulsions have been characterized as oil-external or water-external and generally are considered miscible with the crude oil in the formation and the water which displaces it, at least when initially injected.
More recent activities have suggested the use of microemulsion systems which are immiscible with the formation crude oil and/or formation water upon injection but which exhibit extremely low interfacial tensions with the crude oil and formation water (see for example, U.S. Pat. No. 3,885,628, Reed et al., issued May 27, 1975). While microemulsion flooding appears to have promise in recovering oil, a need exists for techniques to more precisely define the characteristics of the microemulsions and to more accurately design a microemulsion system for application under actual reservoir conditions.